Phenol is a basic commodity chemical with many end uses. Most of the phenol manufactured is prepared from isopropyl benzene, hereafter referred to as cumene. The reaction sequence is short and entails the following steps:
1. Air oxidation of cumene to give cumene hydroperoxide. ##STR1##
2. Acid cleavage of the hydroperoxide to provide phenol and acetone. ##STR2##
The phenol and acetone are separated and each one purified to the degree necessary to satisfy its ultimate use. As is readily observed, for every mole of phenol that is produced, a mole of acetone is also theoretically produced. Acetone is also a commodity chemical with various end uses. Although not shown in the schematic equations, there are by-products formed as well which must be removed to various degrees depending upon the end use of the phenol or acetone. Additionally, other by-products are formed by various concentrating methods and processing conditions utilized after the cleavage of the cumene hydroperoxide. The by-products include dimethylbenzyl alcohol, .alpha.-methylstyrene, cumylphenol, mesityl oxide, hydroxy acetone, benzene, toluene, ethylbenzene, dimers and higher polymers of various components including .alpha.-methylstyrene.
Various of these heavier tar like materials, for example the above mentioned polymers, generally known as "heavy ends", are removed as residues from the bottoms of a distillation tower utilized to purify the phenol. Since many of these "heavy ends" are made from products desirable for recycle such as cumene and .alpha.-methylstyrene, as well as cumyl phenol, it has become part of the general processing steps of phenol manufacture to break down these heavy ends through, for example, a heat treatment, "cracking", to their individual desirable compounds such as cumene. These compounds are then recycled into the process streams thereby increasing the overall conversion of the process. However in accomplishing this desirable effect, the heat treatment also produced other products which had undesirable effects on product purity and process economics when introduced into the process streams on recycle. The production of these undesirable compounds by the heat treatment has not been appreciated to date. Specifically the cracking of the heavy ends to moieties including benzene, toluene and ethylbenzene places impurities into the acetone which are extremely difficult if not impossible to economically remove during the acetone purification. The presence of benzene in product acetone removes certain end use markets from consideration. Therefore, an acetone product with a substantially reduced benzene content is a desirable goal. The presence of ethylbenzene further downstream in the distillation train brings about poorer process economics when its removal is performed by prior art methods.
Better acetone quality and improved process economics have been achieved by removal of the benzene, toluene and ethylbenzene through the method of this invention. Not only are the advantages of my earlier copending application Ser. No. 482,298, filed Apr. 5, 1983 and herein incorporated by reference maintained, but additional advantages are observed. Efficient removal of ethylbenzene occurs with very little loss of cumene. Moreover, a more efficient separation of cumene from butyl benzenes surprisingly occurs further down the distillation train as a result of the fact that most of the ethylbenzene has already been removed. Surprisingly it has also been noticed that compounds which deleteriously affect phenol quality according to the sulfonation color test can also be removed from the process by the method of this invention. Previously such compounds had been removed in the phenol purification procedures at a more disadvantageous time, thereby resulting in more product loss and poorer energy utilization. These positive results are achieved with very little loss in overall cumene, the process starting material. In fact, cumene recovery is actually increased and provides further economic justification for an additional separation procedure.